<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    html{
      font-size: 10px;
    }
    .box {
      width: 100rem;
      height: 200px;
      background-color: pink;
    }
    @media screen and (max-width: 768px) {
      html{
        font-size: 6px;
      }
    }

    /* @media screen and (max-width: 768px) {
      .box {
        width: 500px;
      }
    }
    @media screen and (max-width: 480px) {
      .box {
        width: 300px;
      }
    } */
  </style>
</head>
<body>
  <div class="box"></div>
</body>
</html>